The present invention relates to a control system for an electromagnetic clutch for a transmission of a motor vehicle, and more particularly to a system for providing an optimum torque capacity of the clutch in a starting mode of the vehicle and in a lockup engagement mode.
Various systems for controlling the electromagnetic clutch for a continuously variable belt-drive transmission have been proposed by the applicant. Generally, the electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes such as a starting mode of a vehicle, a reverse excitation mode, a drag mode, and a mode of lockup engagement. One of the modes is selected in accordance with a position of a selector lever and driving conditions to control the electromagnetic clutch.
For example, Japanese Patent Laid Open No. 60-139540 (U.S. Pat. No. 4,669,591) discloses a control system for an electromagnetic clutch. In this system the torque capacity of the clutch is controlled to increase with increase of engine speed in the starting mode, while in the lockup engagement mode, torque capacity is controlled in accordance with engine torque which is determined by the engine speed and engine load. In a transient state mode between the starting mode and lockup engagement mode, the torque capacity gradually increases to smoothly change the operational modes. Since the torque capacity increases with the increase of the engine speed, the starting characteristic can be desirably provided. However, there is a discontinuous changing point in the control operation during the transient state mode, which has an unfavorable influence on the control.
The operation of the control system in the starting and lockup engagement modes is described in detail with reference FIGS. 3a, 5a and 5b. The relationship between the engine torque Te and torque capacity Tc of the clutch can be generally expressed as follows. EQU Te-Tc=Ie.multidot.d.omega.e/dt
where Ie is a moment of inertia of the engine and .omega.e is an angular velocity of the crankshaft of the engine. When the engine torque and torque capacity are equal, the engine speed is constant (d.omega.e/dt=0).
In the starting mode, the torque capacity Tc is controlled as a function of the engine speed Ne as shown in FIG. 3a and by a line Tc in FIG. 5a. The characteristic of the engine torque Te also changes in dependency on the engine speed Ne at each throttle opening degree, as shown by lines Te.alpha. Te.beta. which show the engine torque characteristics at throttle opening degrees .alpha. and .beta., respectively. At the throttle opening degree .alpha., the torque capacity Tc is equal to the engine torque Tea at a point A when the engine speed is Nea. When the throttle is further opened to the opening degree .beta., the engine torque increases from Tea to Teb at a point B, since the engine speed does not quickly increase. After that, the engine speed increases to Nec at a speed in accordance with the following equation. EQU d.omega.e/dt=(Teb-Tca)/Ie
As the engine speed increases, the engine torque Teb decreases along the line Te.beta. toward an engine torque Tec at a point C. At the same time, the torque capacity increases with increasing the engine speed along the line Tc so as to become equal to the engine torque at the point C. Thus, when the engine torque is increased, the clutch slips and thereafter the torque capacity increases with increasing of engine speed, and finally engages again. Namely, there is a converge operation such as the feedback operation.
However, although the clutch engages appropriately, the slipping of the clutch occurs whenever the engine torque changes. Accordingly, in a high vehicle speed range, increase of the engine torque results in rapid increase of the engine speed so that the engine torque is not effectively transmitted by the slipping of the clutch. Thus, driveability and fuel consumption of the vehicle deteriorate.
On the other hand, in the lockup engagement mode, the relationship among the engine torque Te, torque capacity Tc and torque Tr of running resistance of the vehicle can be represented as follows. EQU Te-Tc=Ie.multidot.d.omega.e/dt EQU Tc-Tr=Ib.multidot.d.omega.b/dt
where Ib is the moment of inertia of the vehicle and .omega.b is the angular velocity of an output member of the clutch. Since .omega.e=.omega.b in the lockup engagement mode, EQU Tc=(Ib.multidot.Te+Ie.multidot.Tr)/(Ie+Ib)
When the vehicle is in a steady state, the torque Tr of running resistance is equal to the engine torque Te so that the clutch torque Tc is equal to the engine torque Te.
Referring to FIG. 5b, when the throttle opening degree changes from .alpha. at a point A, where engine torque and torque capacity are equal to each other, to .beta., the engine torque increases from Tea to Teb. Accordingly, the torque capacity increases from Tca to Tcb. Thus, engine speed is constant (d .omega.e/dt=0), so that engine torque is transmitted to the transmission system without racing of the engine. However, since the clutch does not slip, the vehicle cannot be smoothly started at the starting mode. Additionally, if the engine torque increases abnormally beyond a predetermined range, the clutch starts to slip. Since the system does not have a converge function, the clutch capacity does not increase, so that the clutch continues to slip.
In order to avoid such disadvantages, Japanese Patent Laid Open No. 59-187118 discloses a system which provides a torque capacity larger than the engine torque. However, during the starting mode, the engine torque is not smoothly transmitted thereby causing the driveability to deteriorate.